stap - Man Page

Whitespace is ignored.  Three forms of comments are supported:

Literals are either strings enclosed in double-quotes (passing through the usual C escape codes with backslashes, and with adjacent string literals glued together, also as in C), or integers (in decimal, hexadecimal, or octal, using the same notation as in C).  All strings are limited in length to some reasonable value (a few hundred bytes). Integers are 64-bit signed quantities, although the parser also accepts (and wraps around) values above positive 2**63.

In addition, script arguments given at the end of the command line may be inserted.  Use $1 ... $<NN> for insertion unquoted, @1 ... @<NN> for insertion as a string literal.  The number of arguments may be accessed through $# (as an unquoted number) or through @# (as a quoted number).  These may be used at any place a token may begin, including within the preprocessing stage.  Reference to an argument number beyond what was actually given is an error.

A simple conditional preprocessing stage is run as a part of parsing. The general form is similar to the cond ? exp1 : exp2 ternary operator:

%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)

The CONDITION is either an expression whose format is determined by its first keyword, or a string literals comparison or a numeric literals comparison.  It can be also composed of many alternatives and conjunctions of CONDITIONs (meant as in previous sentence) using || and && respectively. However, parentheses are not supported yet, so remembering that conjunction takes precedence over alternative is important.

If the first part is the identifier kernel_vr or kernel_v to refer to the kernel version number, with ("2.6.13-1.322FC3smp") or without ("2.6.13") the release code suffix, then the second part is one of the six standard numeric comparison operators <, <=, ==, !=, >, and >=, and the third part is a string literal that contains an RPM-style version-release value.  The condition is deemed satisfied if the version of the target kernel (as optionally overridden by the -r option) compares to the given version string.  The comparison is performed by the glibc function strverscmp. As a special case, if the operator is for simple equality (==), or inequality (!=), and the third part contains any wildcard characters (* or ? or [), then the expression is treated as a wildcard (mis)match as evaluated by fnmatch.

If, on the other hand, the first part is the identifier arch to refer to the processor architecture (as named by the kernel build system ARCH/SUBARCH), then the second   part is one of the two string comparison operators == or !=, and the third part is a string literal for matching it.  This comparison is a wildcard (mis)match.

Similarly, if the first part is an identifier like CONFIG_something to refer to a kernel configuration option, then the second part is == or !=, and the third part is a string literal for matching the value (commonly "y" or "m").  Nonexistent or unset kernel configuration options are represented by the empty string.  This comparison is also a wildcard (mis)match.

If the first part is the identifier systemtap_v, the test refers to the systemtap compatibility version, which may be overridden for old scripts with the --compatible flag.  The comparison operator is as is for kernel_v and the right operand is a version string.  See also the Deprecation section below.

If the first part is the identifier systemtap_privilege, the test refers to the privilege level that the systemtap script is compiled with. Here the second part is == or !=, and the third part is a string literal, either "stapusr" or "stapsys" or "stapdev".

If the first part is the identifier guru_mode, the test refers to if the systemtap script is compiled with guru_mode. Here the second part is == or !=, and the third part is a number, either 1 or 0.

If the first part is the identifier runtime, the test refers to the systemtap runtime mode. See Alternate Runtimes below for more information on runtimes. The second   part is one of the two string comparison operators == or !=, and the third part is a string literal for matching it.  This comparison is a wildcard (mis)match.

Otherwise, the CONDITION is expected to be a comparison between two string literals or two numeric literals.  In this case, the arguments are the only variables usable.

The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens (possibly including nested preprocessor conditionals), and are passed into the input stream if the condition is true or false.  For example, the following code induces a parse error unless the target kernel version is newer than 2.6.5:

%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence

The following code might adapt to hypothetical kernel version drift:

probe kernel.function (
  %( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
     %( kernel_vr == "2.6.13*smp" %? "do_page_fault" %:
        UNSUPPORTED %) %)
) { /* ... */ }

%( arch == "ia64" %?
   probe syscall.vliw = kernel.function("vliw_widget") {}
%)

The preprocessor also supports a simple macro facility, run as a separate pass before conditional preprocessing.

Macros are defined using the following construct:

@define NAME %( BODY %)
@define NAME(PARAM_1, PARAM_2, ...) %( BODY %)

Macros, and parameters inside a macro body, are both invoked by prefixing the macro name with an @ symbol:

@define foo %( x %)
@define add(a,b) %( ((@a)+(@b)) %)

   @foo = @add(2,2)

Macro expansion is currently performed in a separate pass before conditional compilation. Therefore, both TRUE- and FALSE-tokens in conditional expressions will be macroexpanded regardless of how the condition is evaluated. This can sometimes lead to errors:

// The following results in a conflict:
%( CONFIG_UPROBE == "y" %?
    @define foo %( process.syscall %)
%:
    @define foo %( **ERROR** %)
%)

// The following works properly as expected:
@define foo %(
  %( CONFIG_UPROBE == "y" %? process.syscall %: **ERROR** %)
%)

The first example is incorrect because both @defines are evaluated in a pass prior to the conditional being evaluated.

Normally, a macro definition is local to the file it occurs in. Thus, defining a macro in a tapset does not make it available to the user of the tapset. Publically available library macros can be defined by including .stpm files on the tapset search path. These files may only contain @define constructs, which become visible across all tapsets and user scripts. Optionally, within the .stpm files, a public macro definition can be surrounded by a preprocessor conditional as described above.

Tapsets or guru-mode user scripts can access header file constant tokens, typically macros, using built-in @const() operator.  The respective header file inclusion is possible either via the tapset library, or using a top-level guru mode embedded-C construct.  This results in appropriate embedded C pragma comments setting.

@const("STP_SKIP_BADVARS")

Identifiers for variables and functions are an alphanumeric sequence, and may include _ and $ characters.  They may not start with a plain digit, as in C.  Each variable is by default local to the probe or function statement block within which it is mentioned, and therefore its scope and lifetime is limited to a particular probe or function invocation.

Scalar variables are implicitly typed as either string or integer. Associative arrays also have a string or integer value, and a tuple of strings and/or integers serving as a key.  Here are a few basic expressions.

var1 = 5
var2 = "bar"
array1 [pid()] = "name"     # single numeric key
array2 ["foo",4,i++] += 5   # vector of string/num/num keys
if (["hello",5,4] in array2) println ("yes")  # membership test

The translator performs type inference on all identifiers, including array indexes and function parameters. Inconsistent type-related use of identifiers signals an error.

Variables may be declared global, so that they are shared amongst all probes and functions and live as long as the entire systemtap session.  There is one namespace for all global variables, regardless of which script file they are found within.  Concurrent access to global variables is automatically protected with locks, see the Safety and Security section for more details.  A global declaration may be written at the outermost level anywhere, not within a block of code.  Global variables which are written but never read will be displayed automatically at session shutdown.  The translator will infer for each its value type, and if it is used as an array, its key types.  Optionally, scalar globals may be initialized with a string or number literal.  The following declaration marks variables as global.  

global var1, var2, var3=4

Global variables can also be set as module options. One can do this by either using the -G option, or the module must first be compiled using stap -p4. Global variables can then be set on the command line when calling staprun on the module generated by stap -p4. See staprun(8) for more information.

The scope of a global variable may be limited to a tapset or user script file using private keyword. The global keyword is optional when defining a private global variable. Following declaration marks var1 and var2 private globals.

private global var1=2
private var2

Arrays are limited in size by the MAXMAPENTRIES variable -- see the Safety and Security section for details.  Optionally, global arrays may be declared with a maximum size in brackets, overriding MAXMAPENTRIES for that array only. Note that this doesn't indicate the type of keys for the array, just the size.

global tiny_array[10], normal_array, big_array[50000]

Arrays may be configured for wrapping using the '%' suffix.  This causes older elements to be overwritten if more elements are inserted than the array can hold. This works for both associative and statistics typed arrays.

global wrapped_array1%[10], wrapped_array2%

Many types of probe points provide context variables, which are run-time values, safely extracted from the kernel or userspace program being probed.  These are prefixed with the $ character.  The CONTEXT VARIABLES section in stapprobes(3stap) lists what is available for each type of probe point.  These context variables become normal string or numeric scalars once they are stored in normal script variables.  See the Typecasting section below on how to to turn them back into typed pointers for further processing as context variables.  There is some automation to help!

Statements enable procedural control flow.  They may occur within functions and probe handlers.  The total number of statements executed in response to any single probe event is limited to some number defined by the MAXACTION macro in the translated C code, and is in the neighbourhood of 1000.

EXP

Execute the string- or integer-valued expression and throw away the value.

{ STMT1 STMT2 ... }

Execute each statement in sequence in this block.  Note that separators or terminators are generally not necessary between statements.

;

Null statement, do nothing.  It is useful as an optional separator between statements to improve syntax-error detection and to handle certain grammar ambiguities.

if (EXP) STMT1 [ else STMT2 ]

Compare integer-valued EXP to zero.  Execute the first (non-zero) or second STMT (zero).

while (EXP) STMT

While integer-valued EXP evaluates to non-zero, execute STMT.

for (EXP1; EXP2; EXP3) STMT

Execute EXP1 as initialization.  While EXP2 is non-zero, execute STMT, then the iteration expression EXP3.

foreach (VAR in ARRAY [ limit EXP ]) STMT

Loop over each element of the named global array, assigning current key to VAR.  The array may not be modified within the statement. By adding a single + or - operator after the VAR or the ARRAY identifier, the iteration will proceed in a sorted order, by ascending or descending index or value. If the array contains statistics aggregates, adding the desired @operator between the ARRAY identifier and the + or - will specify the sorting aggregate function.  See the Statistics section below for the ones available.  Default is @count. Using the optional limit keyword limits the number of loop iterations to EXP times.  EXP is evaluated once at the beginning of the loop.

foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT

Same as above, used when the array is indexed with a tuple of keys. A sorting suffix may be used on at most one VAR or ARRAY identifier.

foreach ([VAR1, VAR2, ...] in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT

Same as above, where iterations are limited to elements in the array where the keys match the index values specified. The symbol * can be used to specify an index and will be treated as a wildcard.

foreach (VAR0 = VAR in ARRAY [ limit EXP ]) STMT

This variant of foreach saves current value into VAR0 on each iteration, so it is the same as ARRAY[VAR].  This also works with a tuple of keys.  Sorting suffixes on VAR0 have the same effect as on ARRAY.

foreach (VAR0 = VAR in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT

Same as above, where iterations are limited to elements in the array where the keys match the index values specified. The symbol * can be used to specify an index and will be treated as a wildcard.

break, continue

Exit or iterate the innermost nesting loop (while or for or foreach) statement.

return EXP

Return EXP value from enclosing function.  If the function's value is not taken anywhere, then a return statement is not needed, and the function will have a special "unknown" type with no return value.

next

Return now from enclosing probe handler.  This is especially useful in probe aliases that apply event filtering predicates. When used in functions, the execution will be immediately transferred to the next overloaded function.

try { STMT1 } catch { STMT2 }

Run the statements in the first block.  Upon any run-time errors, abort STMT1 and start executing STMT2.  Any errors in STMT2 will propagate to outer try/catch blocks, if any.

try { STMT1 } catch(VAR) { STMT2 }

Same as above, plus assign the error message to the string scalar variable VAR.

delete ARRAY[INDEX1, INDEX2, ...]

Remove from ARRAY the element specified by the index tuple. If the index tuple contains a * in place of an index, the * is treated as a wildcard and all elements with keys that match the index tuple will be removed from ARRAY. The value will no longer be available, and subsequent iterations will not report the element. It is not an error to delete an element that does not exist.

delete ARRAY

Remove all elements from ARRAY.

delete SCALAR

Removes the value of SCALAR.  Integers and strings are cleared to 0 and "" respectively, while statistics are reset to the initial empty state.

Systemtap supports a number of operators that have the same general syntax, semantics, and precedence as in C and awk.  Arithmetic is performed as per typical C rules for signed integers.  Division by zero or overflow is detected and results in an error.

binary numeric operators

* / % + - >> << & ^ | && ||

binary string operators

. (string concatenation)

numeric assignment operators

= *= /= %= += -= >>= <<= &= ^= |=

string assignment operators

= .=

unary numeric operators

+ - ! ~ ++ --

binary numeric, string comparison or regex matching operators

< > <= >= == != =~ !~

ternary operator

cond ? exp1 : exp2

grouping operator

( exp )

function call

fn ([ arg1, arg2, ... ])

array membership check

exp in array
[exp1, exp2, ... ] in array
[*, *, ... ] in array

The scripting language supports regular expression matching. The basic syntax is as follows:

exp =~ regex
exp !~ regex

(The first operand must be an expression evaluating to a string; the second operand must be a string literal containing a syntactically valid regular expression.)

The regular expression syntax supports POSIX Extended Regular Expression features as documented in grep(1) except for subexpression reuse ("\1") functionality.

After a successful match, the contents of the matched string and subexpressions can be extracted using the matched() and ngroups() tapset functions as follows:

if ("an example string" =~ "str(ing)") {
  matched(0) // -> returns "string", the matched substring
  matched(1) // -> returns "ing", the 1st matched subexpression
  ngroups()  // -> returns 2, the number of matched groups
}

The main construct in the scripting language identifies probes. Probes associate abstract events with a statement block ("probe handler") that is to be executed when any of those events occur.  The general syntax is as follows:

probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
probe PROBEPOINT [, PROBEPOINT] if (CONDITION) { [STMT ...] }

Events are specified in a special syntax called "probe points".  There are several varieties of probe points defined by the translator, and tapset scripts may define further ones using aliases.  Probe points may be wildcarded, grouped, or listed in preference sequences, or declared optional.  More details on probe point syntax and semantics are listed on the stapprobes(3stap) manual page.

The probe handler is interpreted relative to the context of each event.  For events associated with kernel code, this context may include variables defined in the source code at that spot.  These "context variables" are presented to the script as variables whose names are prefixed with "$".  They may be accessed only if the kernel's compiler preserved them despite optimization. This is the same constraint that a debugger user faces when working with optimized code.  In addition, the objects must exist in paged-in memory at the moment of the systemtap probe handler's execution, because systemtap must not cause (suppresses) any additional paging. Some probe types have very little context. See the stapprobes(3stap) man pages to see the kinds of context variables available at each kind of probe point.  As of systemtap version 4.3, functions called from the handlers of some probe point types may also refer to context variables.  These are treated as if a clone of that function was inlined into the calling probe handler and $variables evaluated in its context.

Probes may be decorated with an arming condition, consisting of a simple boolean expression on read-only global script variables.  While disarmed (inactive, condition evaluates to false), some probe types reduce or eliminate their run-time overheads.  When an arming condition evaluates to true, probes will be soon re-armed, and their probe handlers will start getting called as the events fire.  (Some events may be lost during the arming interval.  If this is unacceptable, do not use arming conditions for those probes.) Example of the syntax:

probe timer.us(TIMER) if (enabled) {
}

New probe points may be defined using "aliases".  Probe point aliases look similar to probe definitions, but instead of activating a probe at the given point, it just defines a new probe point name as an alias to an existing one. There are two types of alias, i.e. the prologue style and the epilogue style which are identified by "=" and "+=" respectively.

For prologue style alias, the statement block that follows an alias definition is implicitly added as a prologue to any probe that refers to the alias. While for the epilogue style alias, the statement block that follows an alias definition is implicitly added as an epilogue to any probe that refers to the alias.  For example:

probe syscall.read = kernel.function("sys_read") {
  fildes = $fd
  if (execname() == "init") next  # skip rest of probe
}

defines a new probe point syscall.read, which expands to kernel.function("sys_read"), with the given statement as a prologue, which is useful to predefine some variables for the alias user and/or to skip probe processing entirely based on some conditions.  And

probe syscall.read += kernel.function("sys_read") {
  if (tracethis) println ($fd)
}

defines a new probe point with the given statement as an epilogue, which is useful to take actions based upon variables set or left over by the the alias user.  Please note that in each case, the statements in the alias handler block are treated ordinarily, so that variables assigned there constitute mere initialization, not a macro substitution.

Aliases can also be defined to include both a prologue and an epilogue.

probe syscall.read = kernel.function("sys_read") {
  fildes = $fd
  if (execname() == "init") next
},{
  if (tracethis) println ($fd)
}

An alias is used just like a built-in probe type.

probe syscall.read {
  printf("reading fd=%d\n", fildes)
  if (fildes > 10) tracethis = 1
}

Probes with an alias can make use of the @probewrite predicate. This check is used to detect whether a script variable or target variable has been written to in the probe handler body.

@probewrite(var)

expands to 1 iff var has been written to in the probe handler body, otherwise it expands to 0.

In the following example, @probewrite(var) expands to 1 because var has been written to in the probe handler body and consequently, the conditional statement will run.

probe foo = begin { var = 0 }, { if (@probewrite(var)) println(var) }

probe foo {
  var = 1
}

Systemtap scripts may define subroutines to factor out common work. Functions take any number of scalar (integer or string) arguments, and must return a single scalar (integer or string).  An example function declaration looks like this:

function thisfn (arg1, arg2) {
   return arg1 + arg2
}

Note the general absence of type declarations, which are instead inferred by the translator.  However, if desired, a function definition may include explicit type declarations for its return value and/or its arguments.  This is especially helpful for embedded-C functions.  In the following example, the type inference engine need only infer type type of arg2 (a string).

function thatfn:string (arg1:long, arg2) {
   return sprint(arg1) . arg2
}

Functions may call others or themselves recursively, up to a fixed nesting limit.  This limit is defined by the MAXNESTING macro in the translated C code and is in the neighbourhood of 10.

Functions may be marked private using the private keyword to limit their scope to the tapset or user script file they are defined in. An example definition of a private function follows:

private function three:long () { return 3 }

Functions terminating without reaching an explicit return statement will return an implicit 0 or "", determined by type inference.

Functions may be overloaded during both runtime and compile time.

Runtime overloading allows the executed function to be selected while the module is running based on runtime conditions and is achieved using the "next" statement in script functions and STAP_NEXT macro for embedded-C functions. For example,

function f() { if (condition) next; print("first function") }
function f() %{ STAP_NEXT; print("second function") %}
function f() { print("third function") }

During a functioncall f(), the execution will transfer to the third function if condition evaluates to true and print "third function". Note that the second function is unconditionally nexted.

Parameter overloading allows the function to be executed to be selected at compile time based on the number of arguments provided to the functioncall. For example,

function g() { print("first function") }
function g(x) { print("second function") }
g() -> "first function"
g(1) -> "second function"

Note that runtime overloading does not occur in the above example, as exactly one function will be resolved for the functioncall. The use of a next statement inside a function while no more overloads remain will trigger a runtime exception Runtime overloading will only occur if the functions have the same arity, functions with the same name but different number of parameters are completely unrelated.

Execution order is determined by a priority value which may be specified. If no explicit priority is specified, user script functions are given a higher priority than library functions. User script functions and library functions are assigned a default priority value of 0 and 1 respectively. Functions with the same priority are executed in declaration order. For example,

function f():3 { if (condition) next; print("first function") }
function f():1 { if (condition) next; print("second function") }
function f():2 { print("third function") }

Since the second function has highest priority, it is executed first. The first function is never executed as there no "next" statements in the third function to transfer execution.

There are a set of function names that are specially treated by the translator. They format values for printing to the standard systemtap output stream in a more convenient way (note that data generated in the kernel module need to get transferred to user-space in order to get printed).

 The sprint* variants return the formatted string instead of printing it.

print, sprint

Print one or more values of any type, concatenated directly together.

println, sprintln

Print values like print and sprint, but also append a newline.

printd, sprintd

Take a string delimiter and two or more values of any type, and print the values with the delimiter interposed.  The delimiter must be a literal string constant.

printdln, sprintdln

Print values with a delimiter like printd and sprintd, but also append a newline.

printf, sprintf

Take a formatting string and a number of values of corresponding types, and print them all.  The format must be a literal string constant.

The printf formatting directives similar to those of C, except that they are fully type-checked by the translator:

%b

Writes a binary blob of the value given, instead of ASCII text.  The width specifier determines the number of bytes to write; valid specifiers are %b %1b %2b %4b %8b.  Default (%b) is 8 bytes.

%c

Character.

%d,%i

Signed decimal.

%m

Safely reads kernel (without #) or user (with #) memory at the given address, outputs its content.  The optional precision specifier (not field width) determines the number of bytes to read - default is 1 byte.  %10.4m prints 4 bytes of the memory in a 10-character-wide field.  Note, on some architectures user memory can still be read without #.

%M

Same as %m, but outputs in hexadecimal.  The minimal size of output is double the optional precision specifier - default is 1 byte (2 hex chars).  %10.4M prints 4 bytes of the memory as 8 hexadecimal characters in a 10-character-wide field.   %.*M hex-dumps a given number of bytes from a given buffer.

%o

Unsigned octal.

%p

Unsigned pointer address.

%s

String.

%u

Unsigned decimal.

%x

Unsigned hex value, in all lower-case.

%X

Unsigned hex value, in all upper-case.

%%

Writes a %.

The # flag selects the alternate forms.  For octal, this prefixes a 0.  For hex, this prefixes 0x or 0X, depending on case.  For characters, this escapes non-printing values with either C-like escapes or raw octal.  In the case of %#m/%#M, this safely accesses user space memory rather than kernel space memory.

Examples:

a = "alice", b = "bob", p = 0x1234abcd, i = 123, j = -1, id[a] = 1234, id[b] = 4567
print("hello")
	Prints: hello
println(b)
	Prints: bob\n
println(a . " is " . sprint(16))
	Prints: alice is 16
foreach (name in id)  printdln("|", strlen(name), name, id[name])
	Prints: 5|alice|1234\n3|bob|4567
printf("%c is %s; %x or %X or %p; %d or %u\n",97,a,p,p,p,j,j)
	Prints: a is alice; 1234abcd or 1234ABCD or 0x1234abcd; -1 or 18446744073709551615\n
printf("2 bytes of kernel buffer at address %p: %2m", p, p)
	Prints: 2 byte of kernel buffer at address 0x1234abcd: <binary data>
printf("%4b", p)
	Prints (these values as binary data): 0x1234abcd
printf("%#o %#x %#X\n", 1, 2, 3)
	Prints: 01 0x2 0X3
printf("%#c %#c %#c\n", 0, 9, 42)
	Prints: \000 \t *

It is often desirable to collect statistics in a way that avoids the penalties of repeatedly exclusive locking the global variables those numbers are being put into.  Systemtap provides a solution using a special operator to accumulate values, and several pseudo-functions to extract the statistical aggregates.

The aggregation operator is <<<, and resembles an assignment, or a C++ output-streaming operation. The left operand specifies a scalar or array-index lvalue, which must be declared global.  The right operand is a numeric expression.  The meaning is intuitive: add the given number to the pile of numbers to compute statistics of.  (The specific list of statistics to gather is given separately, by the extraction functions.)

foo <<< 1
stats[pid()] <<< memsize

The extraction functions are also special.  For each appearance of a distinct extraction function operating on a given identifier, the translator arranges to compute a set of statistics that satisfy it. The statistics system is thereby "on-demand".  Each execution of an extraction function causes the aggregation to be computed for that moment across all processors.

Here is the set of extractor functions.  The first argument of each is the same style of lvalue used on the left hand side of the accumulate operation.  The @count(v), @sum(v), @min(v), @max(v), @avg(v), @variance(v[, b]) extractor functions compute the number/total/minimum/maximum/average/variance of all accumulated values.  The resulting values are all simple integers.  Arrays containing aggregates may be sorted and iterated. See the foreach construct above.

Variance uses Welford's online algorithm.  The calculations are based on integer arithmetic, and so may suffer from low precision and overflow. To improve this, @variance(v[, b]) accepts an optional parameter b, the bit-shift, ranging from 0 (default) to 62, for internal scaling.  Only one value of bit-shift may be used with given global variable.  A larger bitshift value increases precision, but increases the likelihood of overflow.

$ stap -e \
> 'global x probe oneshot { for(i=1;i<=5;i++) x<<<i println(@variance(x)) }'
12
$ stap -e \
> 'global x probe oneshot { for(i=1;i<=5;i++) x<<<i println(@variance(x,1)) }'
2
$ python3 -c 'import statistics; print(statistics.variance([1, 2, 3, 4, 5]))'
2.5
$

Overflow (from internal multiplication of large numbers) may occur and may cause a negative variance result.  Consider normalizing your input data.  Adding or subtracting a fixed value from all variance inputs preserves the original variance.  Dividing the variance inputs by a fixed value shrinks the original variance by that value squared.

Histograms are also available, but are more complicated because they have a vector rather than scalar value. @hist_linear(v,start,stop,interval) represents a linear histogram from "start" to "stop" (inclusive) by increments of "interval".  The interval must be positive. Similarly, @hist_log(v) represents a base-2 logarithmic histogram. Printing a histogram with the print family of functions renders a histogram object as a tabular "ASCII art" bar chart.

probe timer.profile {
  x[1] <<< pid()
  x[2] <<< uid()
  y <<< tid()
}
global x // an array containing aggregates
global y // a scalar
probe end {
  foreach ([i] in x @count+) {
     printf ("x[%d]: avg %d = sum %d / count %d\n",
             i, @avg(x[i]), @sum(x[i]), @count(x[i]))
     println (@hist_log(x[i]))
  }
  println ("y:")        
  println (@hist_log(y))  
}

The counts of each histogram bucket may be individually accessed via the [index] operator.  Each bucket is addressed from 1 through N (for each natural bucket).  In addition bucket #0 counts all the samples beneath the start value, and bucket #N+1 counts all the samples above the stop value.  Histogram buckets (including the two out-of-range buckets) may also be iterated with foreach.

global x
probe oneshot {
  x <<< -100
  x <<< 1
  x <<< 2
  x <<< 3
  x <<< 100
  foreach (bucket in @hist_linear(x,1,3,1))
    // expecting   1 out-of-range-low bucket
    //             3 payload buckets
    //             1 out-of-range-high bucket
    printf("bucket %d count %d\n",
           bucket, @hist_linear(x,1,3,1)[bucket])
}

Once a pointer (see the CONTEXT Variables section of stapprobes(3stap)) has been saved into a script integer variable, the translator attempts to keep the type information necessary to  access members from that pointer.  

The translator attempts to track DWARF typing associated with script variables assigned from addresses of context $variables, @cast or @var operators.  Depending on the complexity of the script code, this association may pass to related variables, so that -> and [] operators may be used on them, just as on the original context variable.  For example:

foo = $param->foo; printf("x:%d y:%d\n", foo->x, foo->y)
printf("my value is %d\n", ($type == 42 ? $foo : $bar)->value)
printf("my parent pid is %d\n", task_parent(task_current())->tgid)

However, if this association heuristic doesn't work for a script,  using the @cast() operator tells the translator how to interpret the number as a typed pointer.

@cast(p, "type_name"[, "module"])->member

This will interpret p as a pointer to a struct/union named type_name and dereference the member value.  Further ->subfield expressions may be appended to dereference more levels. Note that for direct dereferencing of a pointer {kernel,user}_{char,int,...}($p) should be used. (Refer to stapfuncs(5) for more details.) NOTE: the same dereferencing operator -> is used to refer to both direct containment or pointer indirection. Systemtap automatically determines which.  The optional module tells the translator where to look for information about that type. Multiple modules may be specified as a list with : separators.  If the module is not specified, it will default either to the probe module for dwarf probes, or to "kernel" for functions and all other probes types.

Previously up to systemtap version 4.2, "kernel" was inferred if unspecified. Use --compatible=4.2 to activate this default.

The translator can create its own module with type information from a header surrounded by angle brackets, in case normal debuginfo is not available.  For kernel headers, prefix it with "kernel" to use the appropriate build system. All other headers are built with default GCC parameters into a user module. Multiple headers may be specified in sequence to resolve a codependency.

@cast(tv, "timeval", "<sys/time.h>")->tv_sec
@cast(task, "task_struct", "kernel<linux/sched.h>")->tgid
@cast(task, "task_struct",
      "kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask

Values acquired by @cast may be pretty-printed by the $ and $$ suffix operators, the same way as described in the CONTEXT Variables section of the stapprobes(3stap) manual page.

When in guru mode, the translator will also allow scripts to assign new values to members of typecasted pointers.

Typecasting is also useful in the case of void* members whose type may be determinable at runtime.

probe foo {
  if ($var->type == 1) {
    value = @cast($var->data, "type1")->bar
  } else {
    value = @cast($var->data, "type2")->baz
  }
  print(value)
}

When in guru mode, the translator accepts embedded C code in the top level of the script.  Such code is enclosed between %{ and %} markers, and is transcribed verbatim, without analysis, in some sequence, into the top level of the generated C code.  At the outermost level, this may be useful to add #include instructions, and any auxiliary definitions for use by other embedded code.

Another place where embedded code is permitted is as a function body. In this case, the script language body is replaced entirely by a piece of C code enclosed again between %{ and %} markers. This C code may do anything reasonable and safe.  There are a number of undocumented but complex safety constraints on atomicity, concurrency, resource consumption, and run time limits, so this is an advanced technique.

The memory locations set aside for input and output values are made available to it using macros STAP_ARG_* and STAP_RETVALUE. Errors may be signalled with STAP_ERROR. Output may be written with STAP_PRINTF. The function may return early with STAP_RETURN. Here are some examples:

function integer_ops (val) %{
  STAP_PRINTF("%d\n", STAP_ARG_val);
  STAP_RETVALUE = STAP_ARG_val + 1;
  if (STAP_RETVALUE == 4)
      STAP_ERROR("wrong guess: %d", (int) STAP_RETVALUE);
  if (STAP_RETVALUE == 3)
      STAP_RETURN(0);
  STAP_RETVALUE ++;
%}
function string_ops (val) %{
  strlcpy (STAP_RETVALUE, STAP_ARG_val, MAXSTRINGLEN);
  strlcat (STAP_RETVALUE, "one", MAXSTRINGLEN);
  if (strcmp (STAP_RETVALUE, "three-two-one"))
      STAP_RETURN("parameter should be three-two-");
%}
function no_ops () %{
    STAP_RETURN(); /* function inferred with no return value */
%}

The function argument and return value types have to be inferred by the translator from the call sites in order for this to work. The user should examine C code generated for ordinary script-language functions in order to write compatible embedded-C ones.

The last place where embedded code is permitted is as an expression rvalue. In this case, the C code enclosed between %{ and %} markers is interpreted as an ordinary expression value.  It is assumed to be a normal 64-bit signed number, unless the marker /* string */ is included, in which case it's treated as a string.

function add_one (val) {
  return val + %{ 1 %}
}
function add_string_two (val) {
  return val . %{ /* string */ "two" %}
}
@define SOME_STAP_MACRO %( %{ SOME_C_MACRO %} %)
probe begin {
      printf("SOME_C_MACRO has value: %d\n", @SOME_STAP_MACRO);
}

The embedded-C code may contain markers to assert optimization and safety properties.

/* pure */

means that the C code has no side effects and may be elided entirely if its value is not used by script code.

/* stable */

means that the C code always has the same value (in any given probe handler invocation), so repeated calls may be automatically replaced by memoized values.  Such functions must take no parameters, and also be pure.

/* unprivileged */

means that the C code is so safe that even unprivileged users are permitted to use it.

/* myproc-unprivileged */

means that the C code is so safe that even unprivileged users are permitted to use it, provided that the target of the current probe is within the user's own process.

/* guru */

means that the C code is so unsafe that a systemtap user must specify -g (guru mode) to use this.  (Tapsets are permitted and presumed to call them safely.)

/* unmangled */

in an embedded-C function, means that the legacy (pre-1.8) argument access syntax should be made available inside the function. Hence, in addition to STAP_ARG_foo and STAP_RETVALUE one can use THIS->foo and THIS->__retvalue respectively inside the function. This is useful for quickly migrating code written for SystemTap version 1.7 and earlier.

/* pragma:suffix */

Emit the file-level containing C code near the end of the resulting kernel module C source file.

/* unmodified-fnargs */

in an embedded-C function, means that the function arguments are not modified inside the function body.

/* string */

in embedded-C expressions only, means that the expression has const char * type and should be treated as a string value, instead of the default long numeric.

Script level global variables may be accessed in embedded-C functions and blocks. To read or write the global variable var , the /* pragma:read:var */ or /* pragma:write:var */ marker must be first placed in the embedded-C function or block. This provides the macros STAP_GLOBAL_GET_* and STAP_GLOBAL_SET_* macros to allow reading and writing, respectively. For example:

global var
global var2[100]
function increment() %{
    /* pragma:read:var */ /* pragma:write:var */
    /* pragma:read:var2 */ /* pragma:write:var2 */
    STAP_GLOBAL_SET_var(STAP_GLOBAL_GET_var()+1); //var++
    STAP_GLOBAL_SET_var2(1, 1, STAP_GLOBAL_GET_var2(1, 1)+1); //var2[1,1]++
%}

Variables may be read and set in both embedded-C functions and expressions. Strings returned from embedded-C code are decayed to pointers. Variables must also be assigned at script level to allow for type inference. Map assignment does not return the value written, so chaining does not work.

A set of builtin probe point aliases are provided by the scripts installed in the directory specified in the stappaths(7) manual page.  The functions are described in the stapprobes(3stap) manual page.

Integers can be dereferenced from pointers saved as a script integer variables using the @kderef() or @uderef() operators. @kderef() is used for kernel space addresses and @uderef() is used for user space addresses.

@kderef(SIZE, addr)
@uderef(SIZE, addr)

This will interpret addr as a kernel/user address and read SIZE bytes starting at that address. SIZE should be either 1, 2, 4 or 8 bytes.

The value stored within a register can be accessed using the @kregister() or @uregister() operators. @kregister() is used for kernel space registers and @uregister() is used for user space registers. The register of interest is specified using its DWARF number.

@kregister(0)
@uregister(5)

One should avoid killing the stap process forcibly, for example with SIGKILL, because the stapio process (a child process of the stap process) and the loaded module may be left running on the system.  If this happens, send SIGTERM or SIGINT to any remaining stapio processes, then use rmmod to unload the systemtap module.

For the normal linux-kernel-module runtime, to run the kernel objects systemtap builds, a user must be one of the following:

• the root user;
• a member of the stapdev and stapusr groups;
• a member of the stapsys and stapusr groups; or
• a member of the stapusr group.

The root user or a user who is a member of both the stapdev and stapusr groups can build and run any systemtap script.

A user who is a member of both the stapsys and stapusr groups can only use pre-built modules under the following conditions:

• The module has been signed by a trusted signer. Trusted signers are normally systemtap compile-servers which sign modules when the --privilege option is specified by the client. See the stap-server(8) manual page for more information.
• The module was built using the --privilege=stapsys or the --privilege=stapusr options.

Members of only the stapusr group can only use pre-built modules under the following conditions:

• The module is located in the /lib/modules/VERSION/systemtap directory.  This directory must be owned by root and not be world writable.

or

• The module has been signed by a trusted signer. Trusted signers are normally systemtap compile-servers which sign modules when the --privilege option is specified by the client. See the stap-server(8) manual page for more information.
• The module was built using the --privilege=stapusr option.

The kernel modules generated by stap program are run by the staprun program.  The latter is a part of the Systemtap package, dedicated to module loading and unloading (but only in the white zone), and kernel-to-user data transfer.  Since staprun does not perform any additional security checks on the kernel objects it is given, it would be unwise for a system administrator to add untrusted users to the stapdev or stapusr groups.

If the current system has SecureBoot turned on in the UEFI firmware, all kernel modules must be signed.  (Some kernels may allow disabling SecureBoot long after booting with a key sequence such as SysRq-X, making it unnecessary to sign modules.)  There are two ways to sign a systemtap module.  The systemtap compile server can sign modules with a MOK (Machine Owner Key) that it has in common with a client system.  For example:

stap --use-server=HOSTNAME:PORT -e 'SCRIPT'
# If there is no mok key in common with the server's systemtap mok key
# list and the client's mok database then the user is directed by stap
# to invoke: 
sudo mokutil --import signing_key.x509
# then after rebooting the system:
stap --use-server=HOSTNAME:PORT -e 'SCRIPT'
# will use the server to build and sign the module and the module will run
# on the client

Another way to sign modules is to use the stap --sign-module option, which uses a MOK on the client system without using a server. For example:

stap --sign-module -e 'SCRIPT'
# If there is no systemtap mok key in the system mok database
# then the user is directed by stap to invoke:
sudo mokutil --import /home/USER/.systemtap/ssl/server/moks/FINGERPRINT/signing_key.x509
# then after rebooting the system:
stap --sign-module -e 'SCRIPT'
# will sign and run the module

See the following wiki page for more details:

Some kernels do not let systemtap guess whether module module signing is in effect.  On such machines, set the SYSTEMTAP_SIGN environment variable to any value while running stap.

Many resource use limits are set by macros in the generated C code. These may be overridden with -D flags.  A selection of these is as follows:

MAXNESTING

Maximum number of nested function calls.  Default determined by script analysis, with a bonus 10 slots added for recursive scripts.

MAXSTRINGLEN

Maximum length of strings, default 128.

MAXTRYLOCK

Maximum number of iterations to wait for locks on global variables before declaring possible deadlock and skipping the probe, default 1000.

MAXACTION

Maximum number of statements to execute during any single probe hit (with interrupts disabled), default 1000. Note that for straight-through probe handlers lacking loops or recursion, due to optimization, this parameter may be interpreted too conservatively.

MAXACTION_INTERRUPTIBLE

Maximum number of statements to execute during any single probe hit which is executed with interrupts enabled (such as begin/end probes), default (MAXACTION * 10).

MAXBACKTRACE

Maximum number of stack frames that will be be processed by the stap runtime unwinder as produced by the backtrace functions in the [u]context-unwind.stp tapsets, default 20.

MAXMAPENTRIES

Maximum number of rows in any single global array, default 2048. Individual arrays may be declared with a larger or smaller limit instead:

global big[10000],little[5]

or denoted with % to make them wrap-around (replace old entries) automatically, as in

global big%

or both.

MAPHASHBIAS

The number of powers-of-two to add or subtract from the natural size of the hash table backing each global associative array.  Default is 0.  Try small positive numbers to get extra performance at the cost of more memory consumption, because that should reduce hash table collisions. Try small negative numbers for the opposite tradeoff.

MAXERRORS

Maximum number of soft errors before an exit is triggered, default 0, which means that the first error will exit the script.  Note that with the --suppress-handler-errors option, this limit is not enforced.

MAXSKIPPED

Maximum number of skipped probes before an exit is triggered, default 100. Running systemtap with -t (timing) mode gives more details about skipped probes.  With the default -DINTERRUPTIBLE=1 setting, probes skipped due to reentrancy are not accumulated against this limit.  Note that with the --suppress-handler-errors option, this limit is not enforced.

MINSTACKSPACE

Minimum number of free kernel stack bytes required in order to run a probe handler, default 1024.  This number should be large enough for the probe handler's own needs, plus a safety margin.

MAXUPROBES

Maximum number of concurrently armed user-space probes (uprobes), default somewhat larger than the number of user-space probe points named in the script. This pool needs to be potentially large because individual uprobe objects (about 64 bytes each) are allocated for each process for each matching script-level probe.

STP_MAXMEMORY

Maximum amount of memory (in kilobytes) that the systemtap module should use, default unlimited.  The memory size includes the size of the module itself, plus any additional allocations.  This only tracks direct allocations by the systemtap runtime.  This does not track indirect allocations (as done by kprobes/uprobes/etc. internals).

STP_OVERLOAD_THRESHOLD, STP_OVERLOAD_INTERVAL

Maximum number of machine cycles spent in probes on any cpu per given interval, before an overload condition is declared and the script shut down.  The defaults are 500 million and 1 billion, so as to limit stap script cpu consumption at around 50%.

STP_PROCFS_BUFSIZE

Size of procfs probe read buffers (in bytes).  Defaults to MAXSTRINGLEN. This value can be overridden on a per-procfs file basis using the procfs read probe .maxsize(MAXSIZE) parameter.

With scripts that contain probes on any interrupt path, it is possible that those interrupts may occur in the middle of another probe handler.  The probe in the interrupt handler would be skipped in this case to avoid reentrance. To work around this issue, execute stap with the option -DINTERRUPTIBLE=0 to mask interrupts throughout the probe handler.  This does add some extra overhead to the probes, but it may prevent reentrance for common problem cases.  However, probes in NMI handlers and in the callpath of the stap runtime may still be skipped due to reentrance.

In case something goes wrong with stap or staprun after a probe has already started running, one may safely kill both user processes, and remove the active probe kernel module with rmmod. Any pending trace messages may be lost.

A member of the groups stapusr and stapsys may use all probe points.

A member of only the group stapusr may use only the following probes:

• begin, begin(n)
• end, end(n)
• error(n)
• never
• process.*, where the target process is owned by the user.
• timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*
• timer.hz(n)

The following scripting language features are unavailable to all unprivileged users:

• any feature enabled by the Guru Mode (-g) option.
• embedded C code.

The following runtime restrictions are placed upon all unprivileged users:

• Only the default runtime code (see -R) may be used.

Additional restrictions are placed on members of only the group stapusr:

• Probing of processes owned by other users is not permitted.
• Access of kernel memory (read and write) is not permitted.

Some command line options provide access to features which must not be available to all unprivileged users:

• -g may not be specified.

• The following options may not be used by the compile-server client:

      -a, -B, -D, -I, -r, -R

The following environment variables must not be set for all unprivileged users:

SYSTEMTAP_RUNTIME
SYSTEMTAP_TAPSET
SYSTEMTAP_DEBUGINFO_PATH

In general, tapset functions are only available for members of the group stapusr when they do not gather information that an ordinary program running with that user's privileges would be denied access to.

There are two categories of unprivileged tapset functions. The first category consists of utility functions that are unconditionally available to all users; these include such things as:

cpu:long ()
exit ()
str_replace:string (prnt_str:string, srch_str:string, rplc_str:string)

The second category consists of so-called myproc-unprivileged functions that can only gather information within their own processes. Scripts that wish to use these functions must test the result of the tapset function is_myproc and only call these functions if the result is 1. The script will exit immediately if any of these functions are called by an unprivileged user within a probe within a process which is not owned by that user. Examples of myproc-unprivileged functions include:

print_usyms (stk:string)
user_int:long (addr:long)
usymname:string (addr:long)

A compile error is triggered when any function not in either of the above categories is used by members of only the group stapusr.

No other built-in tapset functions may be used by members of only the group stapusr.